Generally, a pump is a device to compress or pump fluid, for example, gas or liquid. Such a pump is operable to directly pump fluid, or to push fluid, having a pressure less than an atmospheric pressure, into a high-pressure tank. In accordance with the use purpose thereof, the pump may be called a compressor, a metering pump, a vacuum pump, etc. Herein, all kinds of pumps are simply called a pump.
A pump may be basically classified into a reciprocating pump and a rotary pump. The reciprocating pump is configured such that a piston reciprocates inside a cylinder to pump fluid. The rotary pump is configured such that rotors, gears, and screws rotate to serve as pistons.
Referring to FIG. 1, a conventional reciprocating pump 10 is a type of a capacity pump in which liquid is pumped in accordance with a variation in the volume of a closed space. The conventional reciprocating pump 10 comprises: a cylinder 16 having a suction valve 12 and a discharge valve 14; a reciprocating piston 18; and a connecting rod 20 to convert a rotating motion into a linear reciprocating motion of the piston 18. As the piston 18 linearly reciprocates into or out of the cylinder 16, fluid, for example, gas or liquid, is suctioned into or discharged from the cylinder 16. Specifically, when the piston 18 comes out of the cylinder 16, the internal pressure of the cylinder 16 is lowered, so that the discharge valve 14 is closed and the suction valve 12 is opened. Thereby, fluid received in a tank 22 is suctioned into the cylinder 16 via a suction pipe. In this case, there is no movement of fluid in a discharge pipe 24. Conversely, when the piston 18 enters the cylinder 16, the suction valve 12 is closed, and simultaneously, the fluid received in the cylinder 16 is discharged into the discharge pipe 24 while pushing the discharge valve 14. In this case, there is no movement of fluid in the suction pipe.
The above-described conventional reciprocating pump 10 of a single-acting pump type tends to cause an excessive variation in the flow rate of fluid and much more pulsations. For this reason, a double-acting or differential reciprocating pump has been used to solve the above problem. However, the double-acting or differential reciprocating pump for reducing the variation in the flow rate of fluid requires the use of a large number of cylinders, pistons, and valves, suffering from a complicated structure and failing to effectively reduce such a variation in the flow rate of fluid.
Further, the conventional reciprocating pump 10 has several other problems in that excessive noise and vibrations are inevitably caused by the linear reciprocating motion of the piston 18, the flow of fluid is often intermitted, and the inertia of the valves 12 and 14, the piston 18, and the connecting rod 20 prevents quick operation of the pump 10. To solve these problems, recently, a rotary pump, which is able to rotate quickly with reduced noise and vibrations, has been mainly used.
Referring to FIG. 2, a conventional rotary pump 50 comprises: a cylindrical housing 56 having a suction hole 52 and a discharge hole 54 formed at opposite locations on the circumference of the housing 56; a rotary cylinder 58 to revolve inside the housing 56; a rotary piston 60 to slide in the rotary cylinder 58; a revolving shaft 64 installed in the center of the rotary piston 60; and a rotating shaft 62 installed eccentric to the center of the rotary piston 60.
The rotary pump 50 eliminates the necessity of suction and discharge valves, a crank, and a connecting rod differently from the reciprocating pump 10. Simply, as the rotary cylinder 58 revolves inside the housing 56 along with several elements and the rotary piston 60 revolves about the rotating shaft 62 in the rotary cylinder 58, the volume of an interior space of the rotary cylinder 58 varies in accordance with a full circular motion, resulting in the pumping of fluid. Since the rotary pump 50 exhibits no intermittence in the flow of fluid differently from the above described single acting pump 10, the rotary pump 50 achieves a much more smooth flow of fluid and a high flow rate of fluid superior to even the combination of two single acting pumps. Accordingly, the rotary pump 50 is suitable to pump high-density fluid. Furthermore, the rotary pump 50 has an advantage in that, even if a high pressure is applied, the pressure acts as a torque of the rotating shaft 62 only, exerting no overload to a machine. The rotary pump 50 also completely discharges suctioned fluid, achieving high efficiency.
However, the conventional rotary pump 50 as described above has the following several problems.
The conventional rotary pump 50 suffers from vibrations during rotation since the rotating shaft 62 is connected to the revolving shaft 64 and the rotary piston 60, and the center of mass of both the revolving shaft 64 and the rotary piston 60 deviates from that of the rotating shaft 62. Of course, the vibrations may be completely eliminated by the use of a balance weight. However, this solution results in an increase in the overall volume and weight of the conventional rotary pump 50. Furthermore, although it is true that the conventional rotary pump 50 achieves a much more smooth flow of fluid as compared to the conventional reciprocating pump 10, the convention rotary pump 50 inevitably confronts an intermittence in the flow of fluid, too. Thus, the rotary pump 50 is not free from a rough flow of fluid and the generation of pulsations.
Further, although the conventional rotary pump 50 does not require the suction valve 12 and the discharge valve 14 of the reciprocating pump 10, the suction hole 52 and the discharge hole 54 at opposite sides of the housing 56 are closed or opened in accordance with the rotation of the rotary cylinder 58, causing various operational problems.
For example, in the case of the conventional rotary pump 50, the suction hole 52 and the discharge hole 54 are simultaneously opened or closed. In a state wherein the interior space of the rotary cylinder 58 is divided into two spaces by the use of the rotary piston 60, if the suction hole 52 is closed, the discharge hole 54 is simultaneously opened, whereas if the discharge hole 54 is closed, the suction hole 52 is simultaneously opened, in association with one of the two spaces. Thus, just when the suction hole 52 and the discharge hole 54 are closed or opened, the pressure of the space is equal to or similar to the pressure of the suction hole 52. Accordingly, when the conventional rotary pump 50 serve as a compression pump or vacuum pump, as soon as the discharge hole 54 is opened, gas at the outside of the suction hole 52 is suddenly suctioned into the lower pressure space of the rotary cylinder 58 through the suction hole 52, but is pushed outward through the suction hole 52 in accordance with the operation of the rotary piston 60. Such a quick backflow of gas results in an increase of pulsations and a rough flow of fluid.
Since there remains no effective space in the rotary cylinder 58 if the discharge hole 54 is closed, the fluid, suctioned through the suction hole 52, can be completely discharged through the discharge hole 54. However, rapidly discharging the fluid through a narrow opening of the discharge hole 54 that is being closed suddenly applies an excessive high-pressure to the rotary piston 60, hindering the rotation of the rotary piston 60. Of course, this problem may be solved by reducing the thickness of upper and lower arched surfaces of the rotary piston 60 to define spaces at upper and lower sides of the rotary piston 60. However, if the fluid is gas, the suctioned gas may partially remain in the spaces rather than being completely discharged, thereby causing backflow when the suction hole 52 is opened. For this reason, when the rotary pump is used as a compression pump, it achieves only a limited compression ratio lower than that of the spaces. On the other hand, when the rotary pump is used as a vacuum pump, the remaining gas results in deterioration in the degree of vacuum.